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92                   METALLURGY OF CAST
through a furnace getting cold, the bed of metal in the hearth will solidify, and then again how, when a furnace is working hot, it will often cut out such a solid mass of iron; but generally, like all workings of mechanical affairs, the evil is prolonged more than the good is hastened, when trouble once begins.
Fig. 19 shows the effect of a chill in a furnace causing metal to solidify around and above the notch. This is one form, and another form, instead of having a chill all around the sides with liquid metal in the middle, may have one side solidified while its opposite is in a fluid state. Solidification of such masses generally occurs by reason of scaffolding, cooling off the furnace, and then letting a mass of chilled stock slip down to the tuyeres or lower into the hearth. There are two forms of such evils resulting from a slip, the first being the solidification of metal as above described, and the other what is called a " lime-set," which is generally caused by reason of a furnace carrying a heavy burden of limestone, and the furnace, becoming cold from '' scaffolding' ' or any other bad working, chills the lime so that it becomes too thick to flush out, and '' sets " in a solid state in the crucible or at the tuyeres.
Furnacemen generally fear a * lime set" more than that of molten metal solidifying, for the latter can be melted away much more readily than the former. Lime-sets have been so serious that furnaces have had to " blow-out" to remove them. A method sometimes employed to gain access through solidified iron, which had closed up tuyeres, or a " notch," so as to prevent its being tapped, is that illustrated by the hydrogen blow-pipe at A, Fig. 19, page 93. As used in this case,ch oftener than when it can be tapped at the regular notch.    It is often surprising how rapidly, as about 75 per cent of the heat generated from the solid fuel is utilized. This is attained where one ton of coke will produce one ton of iron; and Sir........................    2,720     "